XCOFF C_HIDEXT and C_AIX_WEAKEXT classification

XCOFF C_HIDEXT and C_AIX_WEAKEXT classification

If C_HIDEXT and C_AIX_WEAKEXT symbols aren't handled as globals by
coff_classify_symbol then we run into "warning: .. local symbol `some
garbage name' has no section".  These are of course both global
symbols, but C_HIDEXT is like a local in some respects and returning
COFF_SYMBOL_LOCAL for C_HIDEXT keeps nm output looking the same.
Fixes these fails on rs6000-aix5.1:

-FAIL: weakref tests, relocations
-FAIL: weakref tests, global syms
-FAIL: weakref tests, strong undefined syms
-FAIL: weakref tests, weak undefined syms

	* coffcode.h (coff_classify_symbol): Handle C_HIDEXT and
	C_AIX_WEAKEXT.